A modular multilevel cascade converter (MMCC) is known as a next generation transformerless power converter that is easily mounted and is suitable for large-capacity and high-voltage use. The modular multilevel cascade converter is expected to be applied to a static synchronous compensator (STATCOM).
The modular multilevel cascade converter has a feature in cascade connection of converter cells. The modular multilevel cascade converter can be classified into four types on the basis of circuit configurations of a converter cell, and arm connection methods (refer to Non-Patent Literature 2, for example).
One of these types is a static synchronous compensator that uses a modular multilevel cascade converter based on single-star bridge-cells (MMCC-SSBC) that has a feature of good expandability and redundancy (refer to Non-Patent Literature 3, for example). However, because of star-connection (Y-connection), a circulating current cannot be made to flow between phases, and it is difficult to control negative-sequence reactive power.
However, in a static synchronous compensator using a modular multilevel cascade converter based on single-delta bridge-cells (MMCC-SDBC), a circulating current flows in a delta connection line. Accordingly, controlling this circulating current allows an adjustment of negative-sequence reactive power (refer to Non-Patent Literature 4, for example).
FIG. 12 is a circuit diagram of a modular multilevel cascade-type power converter based on single-delta bridge-cells.
A modular multilevel cascade-type power converter 100 based on single-delta bridge-cells includes a delta connection unit inside the power converter 100. Provided in each phase in the delta connection unit is one bridge-cell or a plurality of bridge-cells connected in series to each other. In other words, one bridge-cell or a plurality of bridge-cells connected in series to each other are provided in each of the phases u, v and w in the delta connection unit. In an example illustrated in FIG. 12, in each of the phases in the delta connection unit, three respective bridge-cells 11u-j, 11v-j, and 11w-j (j=1 to 3) are provided. Note that the number of the bridge-cells provided in each phase in the delta connection unit is not limited to three. In other words, one bridge-cell or a plural number of bridge-cells connected in series to each other may be provided in each phase. In FIG. 12, the symbol L expresses a reactor component in each phase in the delta connection unit of the power converter 100.
Each of the bridge-cells 11u-j, 11v-j, and 11w-j (j=1 to 3) includes a direct current (DC) capacitor C, and two semiconductor switch groups that are arranged in parallel with the DC capacitor C. Each semiconductor switch group includes two semiconductor switches connected in series to each other. The semiconductor switch includes a semiconductor switching device that causes a current to flow in one direction through itself at the time of being turned on, and a feedback diode connected in antiparallel to the semiconductor switching device.
In FIG. 12, the symbols VSu, VSv, and VSw indicate phase voltages at respective phases of power supply voltages on the side of a system, the symbols iu, iv, and iw indicate currents (referred to as “power supply current” in the following) at the respective phases. The symbols iuv, ivw, and iwu indicate currents (referred to as “converter current” in the following) each flowing into the respective phases in the delta connection unit of the power converter 100. Further, the symbols vuv, vvw, and vwu indicate output voltages of the respective phases in the delta connection unit of the power converter 100, i.e., inter-line voltages of the output terminals of the power converter 100. The symbols VCju, VCjv, and VCjw (j=1 to 3) indicate voltages of the DC capacitors in each of the bridge-cells 11u-j, 11v-j, and 11w-j. In the following, the elements to which the same reference symbol is assigned in the different drawings are elements having the same function.
For example, in an arc furnace, a large-capacity flicker compensating device capable of performing high-speed control of reactive power of a positive-sequence and a negative-sequence, and capable of performing control of low-frequency active power is used in order to suppress voltage drop or voltage fluctuation caused by the arc furnace. FIG. 13 illustrates a general configuration of a flicker compensating device. Generally, a flicker compensating device 200 is connected in parallel with an arc furnace 500 that is connected in series to a three-phase power supply 300 via an interconnection transformer 400. In FIG. 13, the point of common coupling (PCC) expresses a connection point to a system of the flicker compensating device 200. The symbols p and q each express an instant active power and an instant reactive power sent and received between the PCC and the flicker compensating device 200. A load current iL flowing into the art arc furnace 500 includes not only a positive-sequence active current, but also a positive-sequence reactive current, a negative-sequence reactive current, and a low-frequency active current that induce a voltage flicker. When the flicker compensating device 200 is not provided, these currents appear directly in a power supply current iS so that a voltage flicker is generated. In order to suppress such a voltage flicker, the flicker compensating device 200 generates a compensating current iC.
As such a flicker compensating device, there is a flicker compensating device that uses a thyristor called a static var compensator (SVC) and that has been used before the 1980s (refer to Non-Patent Literature 5, for example). FIG. 14 is a circuit diagram illustrating a flicker compensating device 200 using the SVC. The flicker compensating device 200 using the SVC includes a thyristor controlled reactor (TCR) and a thyristor switched capacitor (TSC). The TCR includes thyristors Tr connected in antiparallel to each other, and a reactor L1 connected in series to the thyristors Tr, as illustrated in FIG. 14A. The TSC includes thyristors Tr connected in antiparallel to each other, and a capacitor C1 and a reactor L2 connected in series to the thristors Tr, as illustrated in FIG. 14B.
In 2000s, a flicker compensating device using a self-excited static synchronous compensator (STATCOM) was put to practical use (refer to Non-Patent Literature 6, for example). FIG. 15 is a circuit diagram illustrating a flicker compensating device described in Non-Patent Literature 6. The flicker compensating device 200 using the STATCOM described in Non-Patent Literature 6 uses a plurality of IEGT converters 202 including a self-arc-extinction device called IEGT, and thereby accomplishing large capacity. The respective IEGT converters 202 are connected to each other in multi-stages via multi-winding coil transformers 201. A power converter including such multi-winding coil transformers has a large capacity, and can control negative-sequence reactive power. Accordingly, this power converter is suitable for use as such a static synchronous compensator.